Capacitance sensors are used to implement a variety of useful functions including touch sensors (e.g., touch pad, touch dial, touch wheel, etc.), determining the presence of an object, accelerometers, and other functions. In general, capacitive sensors are intended to replace mechanical buttons, knobs, and other similar mechanical user interface controls. A capacitive sensor permits eliminating complicated mechanical switches and buttons, providing reliable operation under harsh conditions. Capacitive sensors are widely used in the modern consumer applications, providing new user interface options in the exiting products (cell phones, digital music players, personal digital assistances, etc.).
One class of capacitive sensor uses a charge transfer technique. Referring to FIG. 1, the charge transfer technique charges a sensing capacitor Cx in one phase (switch SW1 closed, switch SW2 open) and discharges the sensing capacitor Cx into a summing capacitor Csum in a second phase (SW1 open, SW2 closed). Switches SW1 and SW2 are operated in a non-overlapping manner repeating the transfer of charge from Cx to Csum.
Capacitance sensor 100 is operated to measure the capacitance of Cx in the following manner. In an initial stage, Csum is reset by discharging Csum by temporarily closing switch SW3. Then, switches SW1 and SW2 commence operating in the two non-overlapping phases that charge Cx and transfer the charge from Cx into Csum. The voltage potential on Csum rises with each charge transfer phase, as illustrated in FIG. 1B. The voltage on Csum can by calculated according to equation 1.
                              V                      C            ⁢                                                  ⁢            sum                          =                              V            dd                    ⁡                      (                          1              -                              ⅇ                                                      -                    N                                    ⁢                                                            C                      ⁢                                                                                          ⁢                      x                                                              C                      ⁢                                                                                          ⁢                      sum                                                                                            )                                              (                  Equation          ⁢                                          ⁢          1                )            where VCsum represents the voltage on Csum, N represents the cycle count, Cx and Csum represent capacitance values, and Vdd represents a power supply voltage. Accordingly, the capacitance of Cx can be determined by measuring the number of cycles (or time) required to raise Csum to a predetermined voltage potential.
The charge transfer method is advantageous due to its relative low sensitivity to RF fields and RF noise. This relative noise immunity stems from the fact that the sensing capacitor Cx is typically charged by a low-impedance source and the charge is transferred to a low-impedance accumulator (i.e., the summing capacitor Csum). However, conventional capacitance sensors have the disadvantage that that voltage on the summing capacitor Csum rises versus time/cycles in an exponential manner (see FIG. 1B and Equation 1). The exponential relationship between the accumulated voltage potential on Csum and the charge transfer time/cycles requires some linearization if the capacitance of Cx is calculated as a function of the voltage potential on Csum after a predetermined time or number of cycles.